1. Field of the Invention
The present invention is directed to a process for reducing the amount of ortho nitro aromatic keto compounds in a mixture containing the same which comprises contacting such mixture with oleum.
2. Description of the Prior Art
Ortho nitro aromatic keto compounds can be found in mixtures wherein their presence is not desired and, obviously, reducing their presence therein, or better still, reducing them entirely, would be highly desirable. For example, diaminobenzophenones, obtained by nitrating benzophenone with nitric acid and thereafter subjecting the resulting dinitrobenzophenones to hydrogenation, can be reacted with a dianhydride, such as 3,4,3',4'-benzophenone tetracarboxylic dianhydride (BTDA) to obtain a polyimide. When the benzophenone is reacted with nitric acid to form the dinitrobenzophenone, a mixture of isomers can be formed, for example, o,o'-dinitrobenzophenone, o,m'-dinitrobenzophenone, o,p-dinitrobenzophenone,m,m'-dinitrobenzophenone,m,p'-dinitrobenzophenone and p,p'-dinitrobenzophenone. Although the m,m'-, m,p'- and p,p'-diamino benzophenones obtained from said mixture will react satisfactorily with BTDA to form desired longchain polyimide resins, the diaminobenzophenones containing an ortho amine substituent will react with BTDA to a lesser extent, resulting in a mixture of long and relatively short polyimide resins. This is believed to be the result of hydrogen bonding between an ortho amine hydrogen and the carbonyl which reduces the basicity of the compound and renders the compound less reactive with BTDA. It would be highly desirable, therefore, to reduce the content of an ortho nitro aromatic keto compound containing the same.